1. Field of the Invention
The present invention relates to a protection circuit and a display device having thereof.
2. Description of the Related Art
In recent years, technology that is used to form thin film transistors using semiconductor thin films (with thicknesses of from several nanometers to several hundreds of nanometers, approximately) that are formed over substrates having an insulating surface has been attracting attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in display devices, in particular, is being pushed.
As a switching element in a display device, a thin film transistor including an amorphous semiconductor film, a thin film transistor including a polycrystalline semiconductor film in which the diameter of the crystal grain is 100 nm or more, or the like is used. As a switching element in a display device, further, a thin film transistor including a microcrystalline semiconductor film is used (see Reference 1: Japanese Published Patent Application No. H4-242724 and Reference 2: Japanese Published Patent Application No. 2005-049832).
In a semiconductor element typified by a thin film transistor, it is one of important objects in a manufacturing process of a semiconductor device that how an electrification phenomenon (charging) which leads to deterioration in an element or a dielectric breakdown is suppressed.
Causes and circumstances of charging are extremely complicated and diverse. A protection circuit using an impedance element such as a diode (a protection diode) or a resistor is used to prevent deterioration or a dielectric breakdown due to charging. When a discharging path is provided by using the protection circuit, charge accumulated in an insulating film can be prevented from being discharged near a semiconductor element or at the intersection of wirings, so that a phenomenon where the semiconductor element deteriorates or is damaged by discharge energy (ESD: electrostatic discharge) can be prevented (Reference 3: Japanese Published Patent Application No. H8-027597).
Further, when the protection circuit is provided, even when noise as well as a signal and power supply voltage is input, a malfunction of a circuit of the next stage due to the noise can be prevented and deterioration or damage of the semiconductor element due to the noise can be prevented.